An RF power amplifier provides the final stage of amplification for a communication signal that has been modulated and converted into an RF signal. Often that RF signal exhibits frequencies in a predetermined RF frequency band licensed by a regulatory agency for a particular use. The RF power amplifier boosts the power of this RF communication signal to a level sufficient so that the signal, when it propagates to an antenna, will be broadcast in such a manner that it will meet the communication goals of the RF transmitter.
Many popular modern modulation techniques, such as CDMA, QAM, OFDM, and the like, require the RF power amplifier to perform a linear amplification operation. In other words, the RF communication signal conveys both amplitude and phase information, and the RF power amplifier should faithfully reproduce both the amplitude and phase content of the RF signal presented to it. While perfect linearity is a goal for any linear RF power amplifier, all linear RF power amplifiers invariably fail to meet it. The degree to which the goal of perfect linearity is missed leads to unwanted intermodulation, distortion, and spectral regrowth. Spectral regrowth refers to an expansion of the bandwidth of an RF communication signal. Governmental regulatory agencies define spectral masks which impose stringent constraints on the spectral emissions from transmitters. Even small amounts of spectral regrowth can cause the transmitter to violate regulatory requirements.
In addition to linearity requirements set through spectral masks, power-added efficiency (PAE) is another parameter of interest to those who design RF transmitters. PAE is the ratio of the RF output power to the sum of the input RF power and the applied bias-signal power. An amplifier that has low PAE wastes power, which is undesirable in any transmitter, but particularly undesirable in battery-powered transmitters because it necessitates the use of undesirably large batteries and/or undesirably frequent recharges. Conventionally, improvements in PAE have been achieved at the expense of linearity. But envelope-tracking (ET) techniques, envelope elimination and restoration (EER) techniques, and hybrids between the two techniques have shown promise for achieving PAE improvements.
Generally, envelope tracking (ET), envelope elimination and restoration (EER), and hybrids of the two refer to techniques for biasing an RF power amplifier using an intentionally applied, time-varying bias signal, where the bias signal varies in time to at least roughly track the envelope of the RF communication signal. The goal of such techniques is to provide a time-varying bias signal to a bias feed network that maintains the bias voltage and current between the conduction nodes of the RF amplifying device at a level no greater than it needs to be to achieve respectably linear amplification.
Those who design RF transmitters understand that different RF power amplifier limitations lead, directly and indirectly, to different types of nonlinearities. One of these nonlinearities results indirectly from an unwanted amplifier-generated low-frequency distortion signal referred to as a video signal or a baseband signal. In particular, RF power amplifiers tend to generate unwanted harmonics of the fundamental RF communication signal being amplified along with the desired amplified fundamental RF communication signal. Filters are often used to remove or otherwise block the harmonics from being broadcast from the transmitter. But the even harmonics have sub-RF byproducts below the fundamental in frequency, extending upward from zero Hz, and these amplifier-generated, sub-RF byproducts represent one form of troublesome distortion.
In some RF power amplifiers additional mechanisms may be present to generate other forms of troublesome sub-RF distortion. For example, the intentionally applied time-varying bias signal used in accordance with an ET, EER, or hybrid biasing scheme is applied through a biasing network to a conduction channel node of an RF power amplifier. The action of the biasing network in combination with the RF power amplifier distorts the time-varying bias signal at the conduction channel node of the RF power amplifier. Typically, the intentionally applied bias signal and its distortion vary in time within roughly the same sub-RF bandwidth as the video signal. These sub-RF, amplifier-generated and bias-generated distortion signals represent sub-RF energy that extends upward from zero Hz, perhaps for a few spans of the bandwidth of the RF communication signal being amplified. While these sub-RF distortion signals are not broadcast from the transmitter, they may nevertheless impede efforts to improve linearity and PAE.
A typical RF amplifier uses an RF amplifying device which is fed a biasing voltage through the biasing network. The amplifier-generated sub-RF distortion signal causes a time-varying voltage to develop across the biasing network, which causes a corresponding and unwanted time-varying voltage modulation of the desired bias voltage applied across conduction nodes of the RF amplifying device. The intentionally applied, time-varying bias signal causes both a wanted time-varying voltage modulation of the bias voltage and an unwanted distortion modulation which occupies substantially the same or a greater bandwidth as the time-varying bias signal. The sub-RF distortion signals can lead to unwanted intermodulations between the sub-RF distortion signals and the RF fundamental signal. The intermodulation causes the RF power amplifier to generate an RF distortion signal which resides in the bandwidth of the fundamental RF signal and extends outside the bandwidth of the fundamental RF signal. This type of RF distortion signal is undesirable because it reduces the signal-to-noise ratio of the transmitted RF signal. But it is highly undesirable due to the spectral regrowth which often must be corrected in order for the transmitter to comply with its spectral mask. Thus, the sub-RF distortion signals cause the RF amplifying device's bias signal to be less stable than desired. Without this sub-RF distortion signal form of bias corruption, linearity and PAE would improve.
Conventional transmitters have addressed the sub-RF distortion problem in at least a few different ways. In one way, the biasing network is configured to implement a series of resonant impedance notches distributed throughout a bandwidth of the sub-RF distortion signal. This technique lowers the overall impedance of the bias network in the sub-RF distortion signal bandwidth, which in turn attenuates the sub-RF distortion signals and reduces the unwanted intermodulation. Unfortunately, this technique does not work well for wide bandwidth signals. One of the requirements of a bias network is to exhibit very high impedance to the amplified fundamental RF signal. For wide bandwidth communication signals it becomes increasingly difficult to configure a biasing network to exhibit adequately low impedance throughout a wide sub-RF distortion signal bandwidth yet exhibit adequately high impedance at the fundamental RF frequency. And, the inclusion of resonant notches in the biasing network is undesirable because it worsens another type of nonlinearity, referred to as “memory effects”. The memory-effect nonlinearities are particularly undesirable because they are difficult to compensate using predistortion techniques which require reasonable computational abilities and consume little power.
In accordance with another technique for addressing the sub-RF distortion signal problem, baseband digital signal processing circuits predict the bias signal corruption that will be caused by the sub-RF distortion signal, then predistort the digital baseband form of the communication signal in a way that will, after upconversion and amplification in the RF power amplifier, compensate for the intermodulation distortion that the sub-RF distortion signal causes. This technique does not rely upon the use of several sub-RF distortion signal bandwidth resonant notches in the biasing network and is effective in reducing the unwanted intermodulation distortion caused by the sub-RF distortion signal. But the bias signal fed to the amplifying device remains less stable in the sub-RF distortion bandwidth than desired.
In accordance with yet another technique for addressing the sub-RF distortion problem, RF power amplifiers have been operated in a significantly de-tuned state where they perform linear amplification regardless of sub-RF distortions that may be present. In other words, the RF amplifiers may be designed so that in theory they are biased and otherwise operated well within the boundaries of linear operation, and not near such boundaries. Any sub-RF distortion may cause actual operation of the RF amplifiers much closer to such boundaries of linear operation, but do not cause overtly non-linear operation. Unfortunately, this technique wastes power and reduces PAE.
Unfortunately, the sub-RF distortion signals are believed to add a component of bias variation which prevents envelope tracking techniques from achieving desired levels of improvement in PAE.
What is needed is an RF transmitter having an RF power amplifier that achieves both improved PAE and improved linearity by stabilizing the bias applied to RF amplifying devices and by avoiding the excessive use of resonant notches in the biasing network.